Perforating process and apparatus



Feb. 21, 1967 w. w JR" ET AL 3,305,018

PERFORATING PROCESS AND APPARATUS Filed April 7. 1964 05E mwu 0 3C WILTON R. MARSHALL TERRY WALKER a I l r z a w I r w a I WILLIAM E. WHITE INVENTORS FIG.

ATTORNEY United States Patent 3,305,018 PERFORATING PROCESS AND APPARATUS William E. White, Jr., Wilton R. Marshall, and Terry Walker, all of Houston, Tex., assignors to Halliburton Company, Duncan, Okla, a corporation of Delaware Filed Apr. 7, 1964, Ser. No. 357,922 6 Claims. (Cl. 166-35) The present invention relates generally to the art of perforating and completing wells, and, moreparticularly to methods and apparatuses for perforating the casing lining Wells.

Explosive perforator equipment and methods wherein a mass (either a projectile or a jet) is projected to accomplish perforation are generally employed to perforate and fluidly communicaate an earth formation through a well bore casing to the bore of a well traversing such formation for the purpose of either producing formation fluid or injecting a desired fluid into the formation. Ideally the perforation should present a large, substantially unrestricted flow area to fluids entering the perforation from the formation or to the fluids passing from the perforation into the formation in the order that the erforation may have a maximized flow capacity with any given differential pressures thereacross.

Prior art explosive perforator apparatuses and methods producing perforations falling short of these ideals have long been well known in the oil industry, and the securing of perforations having desired flow properties has long been considered a problem. The problem arises from the fact that, although commercial perforator equipment is satisfactory from a penetration depth standpoint, the perforation produced may be plugged with detritus from crushed formation and/ or from the perforator equipment itself to an extent such that the perforation presents a flow resistance thereacross which may be greater than the naturally occurring flow resistance of the formation.

In the laboratory evaluation of perforating equipment wherein targets representative of earth formations are perforated, the ratio of the permeability of the target as perforated over the permeability of the tar-get prior to perforation is employed as means of comparison. This ratio is known as well flow index. A Well flow index having a value less than 1 indicates that the permeability or productivity of the target has been reduced by the perforating operation. Well flow index values of unity or greater are sought after.

The American Petroleum Institute has established recommended practices including standard procedures for the evaluation of well perforators in their publication designated API RP 43. These standard procedures provide for the measurement of well flow index under standard conditions in order that various perforator equipment may be compared or evaluated by this means. The standard procedure specifies a back flow through the perforated target for such a time as is required for flow rate stabilization, at which time the post perforation flow rate measurement is taken. The back flow, analogous to production through a single perforation in a well bore, operates to wash considerable detritus from the perforation in the target and, consequently the post perforation fiow rate measurement is considerably higher than if it had been made immediately following perforation. The higher post perforation flow rate results in a higher post perforation target permeability which, in turn, generally raises the well flow index arrived at following the standard procedures.

In actual well completion practices, multiple perforations usually pierce the casing within a given strata. In this situation a few perforations may be washed and cleaned up first and thereafter constitute a preferred path for produced fluids such that the chances are that the remaining perforations will remain plugged. This situation, wherein a significant number of perforations remain plugged, results in low overall completion efficiency because of reduced yield with available differential pressure.

A general object of the present invention is the provision of improved methods and apparatus which, by alleviating the foregoing problems, enable the attainment of more efficient well completions.

A further object of the present invention is provision of improved perforating methods and apparatuses producing perforations more predictable in character, as judged by well flow index measurements made immediately following perforation.

Still another object of the present invention is the provision of improved methods and apparatus for producing borehole perforations which are substantially free of detritus immediately without substantial reliance upon subsequent productive flow through such perforation to clean the same.

A further object of the present invention is the provision of an improved method of perforating wherein the conditions established by the method induce an expulsive flow of formation fluid through the perforation concomitantly with the making thereof.

Another object of the present invention is the provision of methods and apparatus for producing clean perforation in wells adjacent gas bearing formation zones traversed thereby.

Other and further objects of the invention will be obvious upon an understanding of the illustrative embodiment about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon the employment of the invention in practice.

A preferred embodiment of the invention has been chosen for purposes of illustration and description. The preferred embodiment is not intended to be exhaustive nor limit the invention to the precise form disclosed. It is chosen and described in order to best explain the principles of the invention and their application in practical use to thereby enable others skilled in the art to best utilize the invention in various embodiments and modifications as are best adapted to the particular uses contemplated.

In the accompanying drawings:

FIGURE 1 is a cross sectional view, showing casing, cement and earth formation, penetrated by a perforation and illustrating a condition alleviated by the pres-.

ent invention;

FIGURE 2 is a graph illustrating the manner in which perforating characteristics vary with variations in a parameter relationship controlled by the present invention;

FIGURES 3A, 3B and 3C are a sequence of views, similar to FIG. 1, illustrating a theory of mechanical action during the making of a perforation; and

FIGURE 4 is a cross sectional, some-what schematic, view of one apparatus which may be employed in the practice of the method of the invention.

Referring to FIG. 1, a perforation produced by a mass jetted from shaped explosive or jet charge 10 (shown in phantom) is shown in penetrating relationship to a casing wall section 12, a layer of sheath cement 13 and a section of formation 14 bounding a borehole 15. The jetted or other perforating mass, such as a bullet, in entering the formation 14 operates to crush the formation thereahead and laterally thereof. The extent of crushed formation 16 defines a perforation envelope 17 about the axis of the jet or the trajectory of a bullet. The perforation envelope is defined in that, beyond the same, formation substantially retains its original characteristics, i.e.,

Patented Feb. 21, 1967' it is neither generally crushed nor have its grain boundaries been substantially destroyed. Perforation envelope 17, apart from any local radical or longitudinal cracks extending therebeyond, if substantially clean would present the largest flow area obtainable with a given perforator and would yield a maximum well flow index. However, as shown in FIG. 1, a typical perforation produced in accord with prior art practices is likely to be substantially plugged by impacted detritus, comprising crushed formation particles 14 and/ or debris 18 emanating from the perforator equipment, to a degree that the area of the effective flow envelope 1% falls substantially short of the area of the perforation envelope 17. Further, inasmuch as the crushed formation which generally defines the effective flow envelope ordinarily is of lower porosity and permeability than the undisturbed formation 14 beyond the perforation envelope 17, the effective flow envelope 19 may offer a flow restriction considerably greater than might be expected from an area comparison with the perforation envelope 17. This greater flow restriction adversely effects well productivity per perforation.

Continuing with reference to FIG. 1, if the well bore pressure P is greater than the formation pressure P an additional perforation plugging effect comes into play by virtue of borehole fluids entering the effective flow envelope 19 at the time of perforation with the result the detritus 16 and 18 within the perforation is further impacted, and if the well bore fluid is a heavy mud, the perforation may be additionally plugged by filtering action to the point that the well flow index of the perforation is reduced essentially to zero, i.e., the perforation may have no productive capacity.

A large amount of effort, including elaborate and expensive experiments and tests conducted on an industry-Wide basis, has been expended with the aim of solving the pluging problem. Aside from general improvements in com mercial perforator equipment collateraly resulting from the effort, the effort has resulted in little beyond generalities to the effect that the variables investigated did or did not bear statistically significant relation to the problem or that it would appear to be advantageous to perforate a well with a clean fluid in the borehole and, wherever practical, with the bottom hole well bore pressure less than formation pressure either at the time of perforating or within a short time thereafter. These conclusions were based on laboratory tests involving permeable targets saturated with kerosene.

We have observed that perforating practices, based on these conclusions, do not dependably produce desirable perforations, particularly in gas bearing strata wherein perforations are particularly susceptible to plugging.

The present invention is based on a discovery of a relationship of parameters such that, provided these parameters are controlled in the perforating process so as to yield a relationship value in excess of a critical value, the characteristics of cleanliness and well flow index of the perforation produced becomes highly predictable, as compared to perforations made under condition wherein the relationship value is less than this critical value. The relationship is defined as follows:

Relationship PW m where:

Pf is the pressure of formation fluid,

P is the pressure of well bore fluid,

a is the kinematic viscosity of the well bore fluid, and u is the kinematic viscosity of formation fluid.

The manners of variation of well flow index and cleanliness ratio of perforations made in gas bearing Berea sandstone targets as the discovered relationship varies in value is shown in the graph of FIG. 2. The expression cleanliness ratio is simply the depth of the effective flow 4 envelope 19 of a perforation divided by the depth of the perforation envelope 17 of that perforation, and is a convenient mean-s of expression of the degree of or lack of plugging in such perforation. As may be seen from the graph of FIG. 2, as the value of the discover-ed relationship approaches zero the characteristic flow index and cleanliness ratio of the perforation becomes highly unpredictable, varying Widely in value with little or no change in the value of the relationship. On the other hand, if the value of the discovered relationship is maintained in excess of a critical value, unity, the well flow index and cleanliness ratio characteristics of the perforation are substantially assured to be of desirable values.

A general explanation of the beneficial results obtained by the method of perforation of the invention is that when the borehole fiuid parameters as of the time perforating are controlled in value with respect to values of formation fluid parameters such that the value of the discovered relationship is in excess of the critical value, a counter flow force effective to purge the perforation of detritus is brought into play at the instant that fluid communication is established. The counter flow is be lieved to occur commitantly with, i.e., simultaneously with or contemporaneously with or following, the penetrative life of the jet or other perforating mass. Further, the counter flow is believed to impose a shock loading, approaching an explosion in nature, on any detritus plug existing in the perforation, which shock loading is effective to break up such plug and expel the detritus constituents thereof into the borehole. The relatively poor and unpredictable results obtained when perforating under conditions wherein the discovered relationship is less than the critical value, may be because the shock loading, if any, accompanying the act of perforating is too lowvalued for effectiveness.

For full visualization of the general effectiveness of the method of the invention, FIGURES 3A, 3B and 3C show a time sequence, in order stated, of one theory of mechanical progress of a jet 21, emanating from a shaped charge 10', perforating a casing wall, cement and formation, 12', 13' and 14' respectively. For purposes of simplicity and clarity, the concomitance of the counter flow is illustrated in its simultaneous sense as being comprised of arrows 20 (having unshaded heads) representing flow of fluids from formation 14' into the perforation envelope 17, arrows 21' (open heads) representative of jet stream particles which are spent insofar as assisting the jet in further penetration, and arrows 22 (shaded heads) representative of formation fluid flow having spent jet stream particles and formation detritus 16 entrained therein. Thus in the theory of mechanical progress illustrated in FIG. 3A-3C, formation fluids represented by arrows 20 enter the perforation envelope 17 as the jet progressively defines the same. As the formation fluid enters, detritus 16, as well as spent particles of the jet stream, is entrained therein. Any remaining unspent energy of such entrained jet stream particles is added to the energy of the arrows 22 of the counter flow. Thus the counter flow operates to scour the envelope 17 and progressively purge the same of detritus as the envelope is established. The counter flow with the entrained detritus is expelled through the perforation of casing 12 into the borehole. Although jet stream 21 is shown to be still effective to extend the perforation somewhat deeper than is shown in FIG. 3C, it will be apparent from FIG. 3C that the effective flow envelope of the perforation is substantially the perforation envelope 1'7 inasmuch as little, if any, detritus is present on the walls of such envelope to inhibit flow therethrough.

The method of securing clean perforations characterized by high well flow indices may be practiced by ascertaining the pressure condition and kinematic viscosity of formation fluids in advance of perforating, as by formation fluid sampling techniques, and then suitably providing borehole fluid having pressure and kinematic viscosity characteristics such that the value of the discovered relationship referred to above is in excess of the critical value at the level of such planned perforation and at the instant thereof. In addition to the general practice of the method just now described, the method of the invention may be performed locally within an uncontrolled well bore environment by providing the necessary well bore" fluid condition through the agency of suitable apparatus, such as is illustrated in FIG. 4.

The apparatus of FIG. 4 includes a body 30 adapted to be lowered in a case borehole 31 opposite a formation zone to be penetrated by means of a wireline 32 suspending the body from the earths surface.

The upper portion of the body 30 includes a pistoncylinder fluid pressure generator section 34 which is adapted to generate within the hydraulic system of the apparatus a super hydrostatic pressure, i.e., a pressure higher than that due to the column of fluid normally present within the borehole. The super hydrostatic pressure is generated when borehole fluid is admitted to the upper or low pressure end of the pressure generator section through a port 38 pursuant to electrical signal communicated from the earths surface over the wireline 32. Although the port 38 is shown to be open, it is normally blocked by a port plug 39 adapted to be expelled therefrom through the agency of any suitable explosive element 40 (shown in phantom) which may be an ordinary blasting cap, for example. Although the pressure generator section 34 is schematically shown to be of the differential pressure type, disclosed by Chambers in Patent No. 2,674,313, wherein hydrostatic pressure energy of borehole fluid is utilized as a source of power, any other suitable source of power may be employed such as selfcontained gas generator of the type disclosed in commonly assigned Briggs Patent No. 3,090,436, granted May 21, 1963. fluid is admitted to the generator section via port 38, it produces a hydraulic fluid output at a super hydrostatic pressure to other hydraulic system components of the apparatus to actuate the same in the manner described here inafter.

The hydraulic system components supplied by the output of generator section 34, include piston-cylinder type upper and lower actuator elements, 44 and 45 respectively, which are fluidly interconnected with the generator section 34 by means of a passageway 49 provided within the body 30. Actuator elements 44 and 45, although normally biased toward a retracted position with respect to the body 30, extend laterally of the body 30 to make forcible contact with the wall of cased borehole 31 when supplied with pressurized hydraulic fluid. Once the wall has been contacted by the actuator elements, the actuator elements function to urge the body 30 toward an opposite- 1y disposed portion of the wall of cased borehole 31.

The hydraulic system of the apparatus also includes a compartment 50 normally containing a gas at substantially atmospheric pressure. Compartment 50 is adapted for fluid connection with the output of generator section 34 via a normally closed valve 51 when the same is actuated under control from the earths surface. Although valve 51 may be of any suitable remotely controllable type such as is disclosed in commonly assigned McMahan Patent No. 2,982,130, granted May 2, 1961, for example, a valve of the type disclosed in the commonly assigned copending application of Ernest H. Purfurst for 'Fluid Handling System and Apparatus, Serial No. 211,980, filed July 24, 1962, now Patent No. 3,254,661, may be preferred. Valve 51 controls the retraction of actuator elements 44 and 45 by dumping the high pressure hydraulic fluid in the system into compartment 50 which is of sufficient volume to receive the complete hydraulic fluid output of generator section 34. Thus when valve 51 is opened, the pressure within the hydraulic system is dropped substantially to atmospheric pressure which, in turn, permits the retraction of the actuator elements 44 When the hydrostatic pressure of boreholev 6 and 45 by the force of well bore fluid acting on the effective end areas of the extended portions thereof. When valve 51 is opened the generator section will go full stroke under the influence of borehole fluid pressure and the excess hydraulic fluid produced therefrom will be received into compartment 50.

As illustrated, actuator elements 44 and 45 are spaced apart along the body 30. Within the space thus provided between the actuator elements in the body 30, there is provided a chamber 60 which functions as a carrier space for an explosive perforator element, such as the shaped charge 10" illustrated, as well as functioning to provide a local borehole fluid environment having desired propeties at the instant of perforation. Such desired properties would, of course, be those of pressure and kinematic viscosity which, in the discovered relationship, produce a value in excess of the critical. Thus the borehole environment is controllable for purposes of carrying out the method of the invention.

To this end, chamber 60 is preferably filled with a low pressure gas, say air at atmospheric pressure, and is of a volume such that it may contain any gases evolving from the shaped charge 10" when the same is fired at a pressure suitably low for the practice of the method of the invention. By way of example, it has been found experimentally that if RDX is employed as the principal explosive in a shaped charge, such as 10", and if chamber 60 is of a size providing approximately five cubic inches of atmospheric pressure gas volume per gram of RDX employed, then the gas generated upon the firing of the shaped charge will produce a pressure within the chamber on the order of 300 pounds per square inch. This pressure value together with the kinematic viscosity of the combustion products is appropriate for the practice of the method of the invention in relation to many formation fluid situations. Of course, the total volume of the chamber 60 should be adequate to receive all the detritus and formation fluid constituting the counter flow. The volume of chamber 60 may be reduced or increased to respectively increase or reduce the pressure produced therein subsequent to the firing of the explosive. A simple inverse proportionality relationship has been found to predict pressure P with suflicient accuracy for the successful practice of the method with the illustrated apparatus. Thus if the chamber 60 is increased in size to where it provides 10 cubic inches of volume per gram of RDX, then the pressure P in the chamber will be reduced to approximately pounds per square inch.

A combination sidewall sealing and port plug assembly 65 is mounted on the body 30 between actuator elements 44 and 45 in a position such that a donut-like seal 67 thereof is forced against the borehole wall responsive to movement body 30 by the actuator elements 44 and 45. The seal 65 is adapted to isolate an area of casing wall to be perforated from the general fluid environment of the borehole external to the body 30. The assembly 65 also includes a port plug 68 which extends in sealed relation through the body 30 into the chamber 60. The portion of port plug 68 which extends within chamber 60 is configured to receive the conically concave end of the shaped charge 10", and to position the charge so that the jet or perforating axis, defined by the axis of theconcave end, coincides with the axis of a perforating passageway 69 in the port plug 68. The jet. or perforating axis extends through the perforating passageway 69 and the isolated area defined by the donut-like seal 65, as. well as through a thin wall 70 normally closing the perforating passageway 69. The Wall 70 is adapted to be easily breached by the jet, comprised of a metallic mass 71 initially lining the charge concavity, projected from the chamber 60 upon the firing of the shaped charge.

In carrying out the method of perforation in the accordance with the present invention, apparatus would be chosen having a chamber 60 having a volume such that,

after accommodating the gases from the explosive em-- ployed, the pressure P in the chamber will be of sufficiently low value in relation to the kinematic viscosity a of the combustion products, as well as in relation to the pressure and kinematic viscosity of the formation fluid (obtained by formation sampling procedures, for example) to yield a value of the discovered relationship in excess of the critical value thereof.

Having selected suitable apparatus in accord with the foregoing, the apparatus would be lowered into the borehole and the fluid environment (usually provided in the borehole for purposes of pressure control and the like) by means of the w-ireline 32 to a depth such that the donutlike seal 67 is located adjacent the particular formation zone intended for perforation. When so positioned, the apparatus would be actuated by firing the blasting cap 40 by electrical signal transmitted from the earths surface over the wireline so that the port plug 39 is expelled from the port 38 to thus admit borehole fluid to generator section 34 to power the same. The generator section 34, as previously brought out, then supplies hydraulic fluid at a super hydrostatic pressure to the actuator elements 44 and 45 which are effective to force the donut-like seal 6-7 toward the borehole wall to isolate an area thereof. Once the borehole wall section to be perforated has been thus isolated from the general environment of borehole fluids, the shaped charge 10" may be fired through the agency of the usual prima cord and blasting cap arrangement activated by an electrical signal communicated from the earths surface over wireline 32. The jet in making the perforation also establishes fluid communication between the perforation envelope made by the jet and the cham- .ber 60 so that the counter flow from the perforation envelope into the chamber 60 may occur in the manner similar to that described in connection with the description of a mechanical theory of the method of producing the clean perforation of the invention.

After the completion of perforation such as that illustrated in FIG. 3C, the apparatus may be withdrawn from the borehole upon the retraction of actuator elements 44 and 45 which hold the apparatus in anchored position during perforating. This is accomplished by opening normally closed valve 51 in response to electrical signal trans mitted thereto over the wireline 32 from the earths surface. The reaction of the actuator elements, described previously, permits withdrawal of the apparatus from the borehole, whereupon the entire operation is completed.

The effectiveness of the perforating method of the present invention as compared to prior art methods and practices may be further seen from the following example wherein a gas well was first completed by conventional perforating techniques and then was later re-completed following the method of the present invention. The gas well as originally completed or perforated was disappointing in its productivity and after a couple of years of marginal production it was re-perforated or re-completed in the same formation zone in accord with the method of the present invention, whereupon the productivity of the Well was raised 500 percent over the productivity attained by the first completion.

Thus, it has been seen that the method of the invention is capable of significantly improving the productivity of wells as compared to the productivity of such wells if completed in accord with conventional prior art methods and techniques. It has further been seen that this improvement in productivity obtains directly from the clean perforation and the relatively high well flow index provided thereby. It has also been seen that the apparatus of the invention provides for practice of the method in fluid laden boreholes where, for one reason or another, the pressure and kinematic viscosity properties of the borehole fluid may not reasonably be controlled in the general sense.

As various changes may be made in the method and constructions herein described without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted .as illustrative and not in any limiting sense.

What is claimed is:

1. The method of perforating a permeable formation zone containing a gassy formation fluid under pressure and traversed by a casing in a cased and cemented borehole containing a column of fluid extending upwardly of said zone and providing a pressure environment for pressure control in said borehole, comprising the steps of: providing a chamber containing a volume of gas at a pressure less than the pressure of said gassy formation fluid within said casing and enveloped by said pressure environment at the instant of perforating; imparting kinetic energy to a mass and directing the same from within said casing in a first direction along an axis to breach the casing wall and cement to produce a flow channel thercthrough and perforate the formation therebeyond; and inducing a flow of said gassy formation fluid along said axis opposite said first direction concomitantly with the movement of said mass in said first direction by communicating said gassy formation fluid with said chamher through said flow channel at the instant of breaching said casing wall and cement by said mass with the volume of said chamber and the kinematic viscosity of gas in said chamber being such that when and as said mass moves through said formation, the relation er PW m is greater than one, where P, is the pressure of said gassy formation fluid,

P is the pressure in said chamber,

,uw is the kinematic viscosity of gas in said chamber, and t, is the kinematic viscosity of said formation fluid whereby said perforation formed by the passage of said mass is substantially purged of detritus when and as said perforation is being formed.

2. The method of perforating a permeable formation zone containing a gassy formation fluid under pressure and traversed by a casing in a cased and cemented borehole containing a column of fluid extending upwardly of said zone and providing a general pressure environment therein which is greater than the pressure of said gassy formation fluid, comprising the steps of: providing a closed chamber containing a volume of gas at a pressure less than the pressure of said gassy formation fluid within said casing adjacent said zone; isolating a space extending between said chamber and an adjacent casing wall from said general pressure environment; perforating said casing from therewithin in a first direction along an axis extending through said space to breach the casing wall and cement to produce a flow channel therethrough and establish fluid communication with said gassy formation fluid therebeyond by producing a perforation in said formation; and inducing a flow of said gassy formation fluid along said axis opposite said first direction by communicating the same with said chamber through said flow channel at the instant of producing said flow channel with the volume of said chamber and the kinematic viscosity of gas in said chamber being such that when and as said formation is being perforated the relationship P P :|[HL Pw (9f is greater than one, where P, is the pressure of said formation fluid,

P is the pressure of gas in said chamber,

a is the kinematic viscosity of gas in said chamber, and is the kinematic viscosity of said formation fluid,

whereby said perforation is substantially purged of detritus when and as said perforation is being formed.

3. The method of perforating a permeable formation zone containing a gassy formation fluid under pressure and traversed by a casing in a cased and cemented borehole, comprising the steps of: providing within said casing a chamber having a pressure less than the pressure of said formation fluid; imparting kinetic energy to a mass and directing the same from within said casing in a first direction along an axis to breach the casing wall and cement to produce a flow channel therethrough and perforate the formation therebeyond; and including a flow of said gassy formation fluid along said axis opposite said first direction concomitantly with the movement of said mass in said first direction by communicating said gassy formation fluid with said chamber through said flow chanel at the instant of breaching said casing wall and cement by said mass with the volume of said chamber and the kinematic viscosity of material in said chamber being such that when and as said mass moves through said formation the relationship l[ PW m is greater than one, where P is the pressure of the formation fluid,

P is the pressure in said chamber,

aw is the viscosity of gas in said chamber, and

# is the kinematic viscosity of said formation fluid,

whereby said perforation formed by the passage of said mass is substantially purged of detritus when and as said perforation is being formed.

4. The method as set forth in claim 3 in which an explosive change is used to impart said kinetic energy to said mass and said pressure in said chamber and said kinematic viscosity of .gas in said chamber being affected by said explosive charge with said volume being sufliciently large to assure the obtainance of said ratio of less than one.

5. Wireline apparatus for perforating a permeable earth formation zone containing a formation fluid under pressure and traversed by a cased borehole containing a column of fluid extending upwardly of said zone providing a pressure environment within said zone greater than the pressure of formation fluid, said apparatu comprising: a body adapted to be lowered within a borehole by means of a Wireline; perforator means including explosive material disposed in said body for perforating said casing along a predetermined axis to establish fluid communication with the formation therebeyond when said explosive material is fired; a compartment in said body providing a volume of low pressure gas about said charge, said compartment being of a size to contain any gases evolving from said explosive material when fired at a pressure less than the pressure of formation fluid; sealing means on said body adapted to isolate said axis from said pressure environment by sealing off a casing volume portion when urged against an area of easing wall; and means on said body for urging said sealing means into engagement with said casing wall said compartment being of sutficiently large volume such that the relationship is greater than unity, where P is the pressure of said formation fluid,

P is the pressure in said compartment when and as said gases are being evolved from said explosive material,

aw is the kinematic viscosity of said gases, and

Mr is the kinematic viscosity of said formation fluid.

6. Wireline apparatus for perforating a permeable earth formation zone containing a formation fluid under pressure and traversed by a cased borehole containing a column of fluid extending upwardly of said zone providing a pressure environment within said casing greater than the pressure of formation fluid, said apparatus comprising: a body adapted to be lowered within said borehole by means of .a Wireline; explosive material in said body comprising at least one shaped charge adapted to be fired pursuant to signal communicated from the earths surface disposed to direct a jet stream to perforate said casing and formation therebeyond; a compartment in said body providing a volume of low pressure gas about said charge, said compartment being of a size to contain any gases evolving from said explosive material, when fired, at a pressure less than the pressure of formation fluid; sealing means on said body adapted to isolate said jet stream from said pressure enviromnent by sealing off an area of casing wall when urged thereagainst; and means on said body for urging said sealing means into engagement with said casing wall said compartment being of sufficiently large volume such that the relationis greater than unity, where P; is the pressure of said formation fluid,

P is the. pressure in said compartment when and as said gases are being evolved from said explosive material,

aw is the kinematic viscosity of said gases, and

a is the kinematic viscosity of said formation fluid.

References Cited by the Examiner UNITED STATES PATENTS 2,884,836 5/1959 Allen -459 X 3,010,517 11/ 1961 Lanmon 175-4.52 3,153,449 10/1964 Lebourg 166--100 X FOREIGN PATENTS 546,812 10/ 1957 Canada.

CHARLES E. OCONNELL, Primary Examiner.

JACOB L. NACKENOFF, Examiner.

D. H. BROWN, Assistant Examiner. 

1. THE METHOD OF PERFORATING A PERMEABLE FORMATION ZONE CONTAINING A GASSY FORMATION FLUID UNDER PRESSURE AND TRAVERSED BY A CASING IN A CASED AND CEMENTED BOREHOLE CONTAINING A COLUMN OF FLUID EXTENDING UPWARDLY OF SAID ZONE AND PROVIDING A PRESSURE ENVIRONMENT FOR PRESSURE CONTROL IN SAID BOREHOLE, COMPRISING THE STEPS OF: PROVIDING A CHAMBER CONTAINING A VOLUME OF GAS AT A PRESSURE LESS THAN THE PRESSURE OF SAID GASSY FORMATION FLUID WITHIN SAID CASING AND ENVELOPED BY SAID PRESSURE ENVIRONMENT AT THE INSTANT OF PERFORATING; IMPARTING KINETIC ENERGY TO A MASS AND DIRECTING THE SAME FROM WITHIN SAID CASING IN A FIRST DIRECTION ALONG AN AXIS TO BREACH THE CASING WALL AND CEMENT TO PRODUCE A FLOW CHANNEL THERETHROUGH AND PERFORATE THE FORMATION THEREBEYOND; AND INDUCING A FLOW OF SAID GASSY FORMATION FLUID ALONG SAID AXIS OPPOSITE SAID FIRST DIRECTION CONCOMITANTLY WITH THE MOVEMENT OF SAID MASS IN SAID FIRST DIRECTION BY COMMUNICATING SAID GASSY FORMATION FLUID WITH SAID CHAMBER THROUGH SAID FLOW CHANNEL AT THE INSTANT OF BREACHING SAID CASING WALL AND CEMENT BY SAID MASS WITH THE VOLUME OF SAID CHAMBER AND THE KINEMATIC VISCOSITY OF GAS IN SAID CHAMBER BEING SUCH THAT WHEN AND AS SAID MASS MOVES THROUGH SAID FORMATION, THE RELATION 